Several chemical components exist in the oxide melt, including ionic species. During crystal growth these are partitioned into the solid phase with their own equilibrium partitioning coefficients. Therefore, they are usually segregated, even at the congruent composition. However, when a compound has a stoichiometric structure at the congruent composition, the equilibrium partitioning coefficients of all chemical components become unity and segregation does not occur. Segregation of ionic species was observed at the binary congruent composition of LiNbO3 because of its non-stoichiometric structure. On the other hand, the segregation of ionic species did not occur at the ternary congruent composition of MgO-doped LiNbO3 (Li2O:Nb2O5:MgO = 45.5:50:4.5). This demonstrates that non-stoichiometry of congruent LiNbO3 was converted to the constitutional stoichiometry at the ternary congruent composition by MgO doping. (C) 2009 Elsevier B.V. All rights reserved.